There is a substantial need for new biomarkers in the study of cystic fibrosis (CF) lung disease. Conventional endpoints, such as rate of FEV1 decline, require prolonged trials and large sample sizes to demonstrate therapeutic efficacy. Ideally such biomarkers would provide a quantitative window to the most basic aspects of CF pathophysiology, allowing for the development and evaluation of therapies prior to large scale clinical trials. The basic defect of CF lung disease occurs in the airways where dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial sodium (ENaC) channels is thought to create an ionic gradient that causes excessive liquid absorption across the epithelium. This results in a dehydrated airway surface liquid (ASL) layer, defective mucociliary clearance, and an increased proclivity for infection and inflammation.